It was shown that O-X-B mode conversion may be optimized by changing the O-mode
insertion angle with respect to the external magnetic field direction: in this case, the O
mode is completely converted into a slow X mode, which under certain conditions is in turn converted to
BWs in the Upper Hybrid Resonance (UHR) layer [1]}. The generated Bernstein waves travel
inside the plasma until they are absorbed at the ECR or at the higher-order cyclotron
harmonics.
A possible design for a microwave injection system producing the O-X-B conversion is shown in Fig. REF : first, an O wave is launched from the outside, with an oblique angle of incidence obtained with a proper orientation of the parabolic mirrors. For an optimal launch angle, it is possible to obtain a correlated optimal parallel refractive index and then a coincidence of the O and X modes at the critical plasma density (cut-off). This
means that both modes have the same phase and group velocities and the power is transferred
without reflections.
Once the X waves are generated, they propagate towards the UHR. Here, the X mode
coincides with the electron Bernstein mode. In the linear description, the X waves are
completely converted into EBWs. This process is called X–B conversion. It should be
noted that the O-X-B process can only take place if the plasma density is above the O-wave
cut-off density.
